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COOS BAY, Ore. - Thousands of jellyfish have washed up on the Oregon Coast in recent days, including Horsfall Beach north of Coos Bay.

The organisms - known as Velella velella, purple sailors or "by-the-wind sailor" jellyfish - are purplish in color but turn clear once they've dried up on land.

In the past, they've shown up in large numbers on beaches, causing a fishy stench.

"One year, like 8 years ago, we had people call them in as an oil spill in Winchester Bay there were so many of them," said Scott Groth, an Oregon Department of Fish and Wildlife shellfish biologist. "They really can get dense."

Groth said Velella velella have stinger cells but are not usually harmful to humans. Still, he advised the public to stay away from them.

Why are they here?

"They move depending on the winds, and with the El Niño looking stronger this year, the winds are a little different," Groth said. "We haven't seem Velella velella wash up in six or seven years or something, but it used to be fairly annual."

ODFW said Velella are offshore residents of the Pacific Ocean.

Winds blowing against its triangular, clear sail move the jellyfish. The sail is set diagonally to the long axis of the animal, ODFW said.

As long as the winds blow gently, Velella tacks at about 45 degree away from a following wind. This keeps the animal offshore.

When winds are strong, the Velella loses its tacking ability and begins spinning and more directly follows the wind.